Rear projection display systems are perhaps the most popular type of large-screen display system available for personal use. These systems have long been used in applications where a self-contained, large-screen display is desired, such as for large-screen television systems. Rear projection display systems generally are available with much larger screen sizes than cathode ray tube (CRT) display systems due to limitations inherent in the manufacture of large cathode ray tubes. More recently, rear projection display systems have also found increased popularity for use in smaller applications, such as monitors. This is at least in part because a rear projection display system will generally have a lesser depth than a CRT monitor of a comparable screen size, and thus have a smaller footprint on a desk or floor.
Rear projection display systems include many individual components that cooperate to display an image for a viewer. For example, a rear projection display system typically includes a body or cabinet with a front side that includes a translucent screen, an image source disposed within the casing, and an optical system. The image source, typically a CRT, a liquid crystal display (LCD), or a digital micromirror device (DMD), produces the image for projection. The optical system includes a plurality of mirrors and lenses configured to direct and focus the image onto the screen.
While many different mirror and lens configurations may be used, all rear projection display systems have a large rear mirror, positioned within the cabinet approximately opposite the screen, off of which light from the image source is reflected toward the screen. Rear projection display systems also often include a smaller front mirror positioned beneath the screen to reflect light from the image source toward the rear mirror. In this arrangement, light first is reflected diagonally upward off of the front mirror toward the back mirror, and then off the back mirror for transmission through the screen. Reflecting the light off of the mirrors increases the path length of the light, thus allowing the projection of a larger image without a corresponding large increase in cabinet depth. A CRT-source rear projection display system may have a depth-to-diagonal ratio (the ratio of cabinet depth to screen diagonal) of as low as about 1:1.2. A LCD- or DMD-source display system may have an even lower ratio, typically approximately 1:2.
Although rear projection display systems offer advantages over other types of displays, they also have some drawbacks. For instance, even with the relatively low depth ratios used in current rear projection display systems, these systems may still be quite heavy. As an example, a system with a 50″ screen diagonal may weigh 200 pounds or more. Furthermore, systems with large screen diagonals may still have a significant depth, and thus occupy an undesirable amount of space in a room.
One possible solution to these problems is to reduce the depth of the cabinet, which reduces both the size and the weight of the display system. Reducing the cabinet depth, however, requires the image source and optics to be reconfigured to compensate for the new cabinet geometry. One way to compensate for reduced cabinet depth is to change the angle of the image source to project the image more vertically upward toward the front mirror. However, in this configuration, the front mirror may need to be larger to reflect the entire cone of light emitted by the light source. Making the front mirror larger may cause the front mirror to obscure partially the path of light between the rear mirror and the lower portion of the screen, and thus harm system performance.
Another way to compensate for reduced cabinet depth is described in Japanese Patent No. JP 11-305337. This solution involves utilizing a screen that reflects light of one polarity but transmits light of the other polarity. Polarized light from the image source is first reflected off of the screen toward a polarization-rotating rear mirror, and then reflected off the rear mirror, which rotates the polarization 90 degrees so that the light can pass through the screen. This system offers the advantage that the small front mirror is omitted, and therefore does not block the screen. However, the use of polarized light lowers the intensity of the projected image by 50% compared to non-polarized light. Thus, the system must use much more power to project an image of the same intensity as one projected with non-polarized light, or otherwise suffer from a low-intensity image.